Funny you give this as an example, that feature is completely unreliable for me. Netflix frequently forgets where I left off, and in the case of TV shows, which episodes I really did see all the way to the end, and which I didn't. Similarly on Prime Video, it's quite unreliable.
Aside from that it does seem that Netflix sometimes 'forgets' if I wait a long time to go back to something, in which case I suppose there can be some sort of cleanup going on. I mean sure I would want to keep these timestamps forever, but probably at a certain scale you want to get rid of old ones as much as it might annoy me.
So a company tht solved it well enough as a backbone tech could be quite valuable on the market
See: multiple ways to skin a cat
Bigger companies are able to reduce the price of storage based on it's value to other features.
Some companies make storage cheaper by going through your data and/or selling it, this goes against our values and we won't do it.
Imagine if you were the Stripe of multi-device sync. Sure maybe end users will never know your name but damn you’d be drowning in money.
Like imagine adding multidevice sync to your product with a simple library instead of everyone cobbling it from scratch and hiring huge teams to make it reliable.